This invention relates to a hydraulic pressure control valve for use in a hydraulic braking system of such as an automotive vehicle and, particularly to so-called brake pressure proportioning valve wherein brake pressure delivered to rear wheel brakes is reduced as compared with brake pressure delivered to front wheel brakes when the pressure exceeds a predetermined level thereby preventing skidding phenomenon of the rear wheels.
Particularly, the hydraulic braking system to which present invention concerns is of the type including a dual type or tandem type master cylinder associated with first and second mutually independent hydraulic lines extending from respective outlet ports of the master cylinder, and respective hydraulic lines are connected to left and right rear wheels respectively and to right and left front wheels respectively or to both front wheels when two independent brake cylinders are provided on each front wheel.
Typically, the pressure proportioning valve mechanism includes a differential pressure piston working in a bore and having different effective pressure receiving areas on opposite ends thereof, an axial passage provided in the piston, and a valve member cooperating with one end of the passage. A chamber facing a small end or an end having a small effective pressure receiving area (inlet chamber) is connected to the outlet port of the master cylinder, and a chamber facing a large end or an end having a large effective pressure receiving area (outlet chamber) is connected to rear wheel. When the pressure supplied to the valve mechanism exceeds a predetermined level, the piston moves toward the small end and the valve member cuts off the communication between two chambers (a cut-off pressure). And when the pressure in the inlet chamber further increases, the piston moves toward the outlet chamber to permit a small amount of liquid flow from the inlet chamber to the outlet chamber and returns to valve closing position. Whereby the pressure in the outlet chamber increases at a reduced rate as compared with the pressure in the inlet chamber.
U.S. Pat. No. 3,970,347 discloses a brake pressure control valve for use with a dual type brake master cylinder and connected to two independent pressure circuits. The valve comprises an elongated housing having an axial bore therein, and two pressure proportioning valve mechanisms are provided in the opposite end portions of the bore in mutually aligned relationship. The valve further includes a failure warning mechanism including a balance piston arranged between the two differential pistons of the pressure proportioning mechanisms and being displaceable to actuate an electric switch when a failure such as a fluid leakage exists in one of pressure circuits.
UK patent application GB No. 2052659A discloses two brake pressure proportioning valve mechanisms mounted on a housing of a tandem type brake master cylinder with the axis of each valve mechanism being perpendicular to the axis of the master cylinder. The detailed construction of the proportioning valve mechanism of the latter particularly with respect to the differential pressure piston and the valve member cooperating with the piston differs substantially from that of U.S. Pat. No. 3,970,347, however, both valve mechanisms operate similarly.
However, these prior art devices have a shortcoming such that even when there exists a failure such as a liquid leakage in either one of pressure circuits the pressure proportioning valve mechanism in the other pressure circuit controls the brake pressure applied to rear wheel similarly to the normal operating condition whereby the braking force acting on the vehicle is reduced about one half and, accordingly, it is required to apply an excessibly large brake applying force on a brake pedal for safely stopping the vehicle.